Process for preparing surface-treated steel strips adapted for electric resistance welding and strips produced by said process

ABSTRACT

Surface-treated steel strips suitable for electric resistance welding are prepared by forming a first layer of iron-nickel alloy on a steel strip, this first layer having a weight ratio of Ni/(Fe+Ni) in the range between 0.02 and 0.50 and a thickness of 10 to 5,000 angstroms, forming a second layer of tin or iron-tin-nickel alloy on the first layer by tin plating to a coating weight of 0.1 to 1 g/m 2  of tin and optionally, causing the tin to reflow. A third layer is formed on the second layer by effecting an electrolytic chromate treatment, this third layer consisting essentially of metallic chromium and hydrated chromium oxide in a total amount of 5 to 20 mg/m 2  calculated as elemental chromium. Preferably, the electrolytic chromate treatment is controlled so that at least 2 mg/m 2  of metallic chromium is present in the third layer.

This invention relates to a process for preparing surface-treated steelstrips adapted for electric resistance welding, and more particularly,to a process for preparing surface-treated steel strips having suchimproved weldability as to permit can bodies to be joined into food cansby electric resistance welding as well as improved corrosion resistanceafter lacquer coating.

BACKGROUND OF THE INVENTION

Among food can-forming materials there have been most widely usedtin-coated steel strips generally called tin plates. In order to jointhe mating edges of a can body, conventional soldering techniques werepreviously used. Because of the toxicity of lead contained inconventional solder, pure tin solder has recently become prevalent. Thepure tin solder, however, has a technical problem in making a jointbecause of inferior wetability during the soldering process and is soexpensive as to create the economic problem of increased manufacturecost.

On the other hand, in recent years, food containers have enjoyed thedevelopment of inexpensive, competitive materials such as polyethylene,aluminum, glass, processed paper and the like. Despite theirsignificantly improved corrosion resistance among others advantages, tinplate cans having expensive tin thickly coated thereon to a coatingweight of as great as 2.8 to 11.2 g/m² require a relatively high cost ofmanufacture and have encountered severe competition.

In order to overcome the above-described drawbacks of tinplate cans,electric resistance welding of can bodies has recently replaced theconventional soldering technique and become widespread. There is theneed for can-forming steel compatible with electric resistance welding.

In addition to tinplate discussed above, tin-free steel of chromium typeis another typical example of conventional can-forming steel. Thetin-free steel is prepared by carrying out an electrolytic chromatetreatment on steel to form a layer of metallic chromium and hydratedchromium oxides on the surface. Since the relatively thick hydratedchromium oxide film on the surface has a relatively high electricresistance, the chromated steel is ineffectively welded to form a weldof insufficient strength and thus unsuitable as welded can-forming steeldespite its economic advantage.

Since other can-forming materials are also inadequate as weldedcan-forming material, a variety of proposals have been made. One exampleis nickel-plated steel, typically "Nickel-Lite" announced by NationalSteel Corporation of the U.S. which is prepared by plating a steel stripwith nickel to a thickness of about 0.5 g/m² followed by a conventionalchromate treatment. Inferior adhesion of lacquer has limited the spreadof this nickel-plated steel.

Another example is "Tin Alloy" announced by Jones & Laughlin SteelCorporation of U.S. This is prepared by thinly coating a steel stripwith tin to a thickness of about 0.6 g/m² and effecting tin fusion orreflow followed by a conventional chromate treatment. Unfortunately,rust resistance and lacquer adhesion are insufficient.

In general, can-forming steel sheets intended for electric resistancewelding are required to exhibit improved weldability and corrosionresistance after lacquering. These requirements will be explained indetail. There must be a proper welding electric current range withinwhich a weld zone having sufficient weld strength is provided at the endof welding without any weld defects such as so-called "splashes". Sincewelded cans are filled with foodstuffs after lacquer coating, theunderlying steel must have sufficient adhesion to lacquer to take fulladvantage of the corrosion prevention of the lacquer film. Furthermore,despite defects unavoidably occurring in a lacquer film, the improvedcorrosion resistance of the underlying steel itself prevents corrosionfrom proceeding.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processfor preparing a surface-treated steel strip suitable for electricresistance welding which is free of the above mentioned drawbacks andfulfills the requirements for welded food cans.

According to a first aspect of the present invention, there is provideda process for preparing a surface-treated steel strip adapted forelectric resistance welding, comprising the steps of

forming a first layer of iron-nickel alloy on a steel strip, said firstlayer having a weight ratio of Ni/(Fe+Ni) in the range between 0.02 and0.50 and a thickness of 10 to 5,000 angstroms,

forming a second layer of tin or iron-tin-nickel alloy on said firstlayer by tin plating to a coating weight of 0.1 to 1 g/m² of tin andoptionally, causing the tin to reflow, and

forming a third layer on said second layer by effecting an electrolyticchromate treatment, said third layer consisting essentially of metallicchromium and hydrated chromium oxide in a total amount of 5 to 20 mg/m²calculated as elemental chromium.

According to a second aspect of the present invention, the electrolyticchromate treatment is controlled such that the following relationships:

2≦X and

5≦X+Y≦20

are met provided that X represents the amount of metallic chromium inthe third layer and Y represents the amount of hydrated chromium oxidein the third layer calculated as elemental chromium, both expressed inmg per square meter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron photomicrograph showing the structure of aniron-tin alloy layer on conventional tinplate;

FIG. 2 is an electron photomicrograph showing the structure of aniron-tin-nickel alloy of a thinly coated tinplate according to thepresent invention; and

FIG. 3 is a diagram showing how the amounts of metallic chromium andhydrated chromium oxide (calculated as chromium) in chromate filmsaffect weldability and corrosion resistance after lacquering.

DETAILED DESCRIPTION OF THE INVENTION

Making extensive investigations on the weldability and corrosionresistance after lacquering of welded can-forming steel plates,particularly thinly coated tinplates, the inventors have found thatweldability remains satisfactory as long as the quantity of a hydratedchromium oxide film does not exceed a certain limit, but corrosionresistance is unsatisfactory within this limit. On the other hand, asthe quantity of a hydrated chromium oxide film increases, corrosionresistance is improved at the sacrifice of weldability, losing theoptimum welding range. If the amount of tin coated is limited, to aslittle as 1 g/m² or less for economic reasons, steel sheets satisfyingweldability and corrosion resistance after lacquering cannot be preparedby merely modifying a conventional tin-plating process so as to controlthe quantity of hydrated chromium oxide.

Continuing further investigation on techniques capable of improvingcorrosion resistance of tinplates other than the chromate treatment, theinventors have found that an iron-tin alloy layer formed by a tin fusionor reflow treatment is improved in corrosion resistance by itself andscarcely soluble in foodstuffs with which cans are filled, for example,fruit juice. Tinplates prepared by conventional techniques, however,have alloy layers containing many interstices as demonstrated by theelectron photomicrograph of FIG. 1 and consequently, such an alloy layeris less effective in protecting the underlying steel.

In order to modify such an alloy layer to improve corrosion resistance,Japanese Patent Application No. 57-200592 discloses a "process forpreparing surface-treated steel strips for forming welded cans",comprising nickel plating followed by annealing to cause the nickel tobe partially or entirely diffused. This process is, however,inconsistent in attainment of corrosion resistance. Some products aresatisfactory, but some are rather impaired in corrosion resistance. Thisprocess does not always provide sufficient corrosion resistance.

The inventors examined why the previously proposed process wasunsatisfactory. Using an ion mass microanalyzer (IMMA), precise analysiswas made on the chemical composition of a nickel-diffused surface layeron steel. It has been found that complete alloying of iron with nickelis essential to improve corrosion resistance. Corrosion resistance isimpaired if part of the nickel is left unalloyed. Even when completealloying is achieved, the proportion of iron and nickel must fall withinthe optimum range in order to provide sufficient corrosion resistance.On the basis of this novel finding, experiments have been repeatedlymade to complete the present invention.

According to the process of the present invention, a surface-treatedsteel strip suitable for electric resistance welding is prepared by thesteps of forming a first layer of iron-nickel alloy on a steel strip,forming a second layer of iron-tin-nickel alloy by depositing tin on thefirst layer in an amount of 0.1 to 1 g/m² and causing the tin to reflow,and forming a third layer of metallic chromium and hydrated chromiumoxide on the second layer by effecting an electrolytic chromatetreatment.

At the outset, the first layer of an iron-nickel alloy may be formed byany one of the following procedures in a currently industriallyacceptable manner.

(a) A steel strip is plated with nickel followed by annealing.

(b) A steel strip is plated with an iron-nickel alloy followed byannealing.

(c) A steel strip is plated with an iron-nickel alloy. These proceduresmay be used alone or in a combination of two or more. These proceduresallow an iron-nickel alloy layer to be formed whose composition variesin the direction of its depth.

The iron-nickel alloy layer itself has improved corrosion resistance andthus greatly contributes to an improvement in the corrosion resistanceof the surface-treated steel strip according to the present invention.According to one aspect of the present invention tin is deposited on thefirst layer followed by a tin reflow treatment to thereby form a secondlayer of an iron-tin-nickel alloy. The resultant dense iron-tin-nickelalloy layer completely covers the first layer and the underlying steel,further improving corrosion resistance. The electron photomicrograph ofFIG. 2 shows the structure of an iron-tin-nickel alloy layer of a thinlycoated tin plate exhibiting improved corrosion resistance. It has beenfound that corrosion resistance is improved to the maximum when thecomposition of the first layer has a weight ratio of Ni/(Fe+Ni) in therange between 0.02 and 0.50. The lower limit of 0.02 is imposed on theweight ratio of Ni/(Fe+Ni) because a significant improvement incorrosion resistance is not obtained below this lower limit. The upperlimit of 0.50 is imposed because the iron-tin-nickel alloy resultingfrom tin fusion or reflow becomes of rough crystal structure andprovides a reduced percent coverage over the underlying steel, resultingin insufficient corrosion resistance. For this reason, the compositionof the first layer of iron-nickel alloy formed on a steel strip islimited to a weight ratio of Ni/(Fe+Ni) in the range of between 0.02 and0.50, and preferably between 0.05 and 0.20.

The thickness of the first layer is limited to 10 to 5,000 angstroms(Å). Thicknesses of less than 10 Å are apparently too small to achievean improvement in corrosion resistance. Iron-nickel alloy layers formedbeyond 5,000 Å thick are so hard and brittle that the iron-nickel alloylayer will crack during mechanical processing of the flange and bead ofa can body to expose the underlying steel, detracting from corrosionresistance. For this reason, the first layer of iron-nickel alloy islimited to a thickness of 10 to 5,000 Å, and preferably 100 to 1,500 Åaccording to the present invention.

A tin plating may be deposited on the first layer of iron-nickel alloyby any desired one of conventional industrial techniques. Tin platingtechniques typically use halide, ferrostan and alkaline baths. Any onemay be selected among these tin plating baths in tin plating the firstlayer according to the present invention while plating conditions neednot be specifically limited. The amount of tin plated should be limitedto the range of 0.1 to 1 g/m². Tin plating to less than 0.1 g/m² cannotfully cover the first layer and is difficult to subsequently yield asecond dense layer of iron-tin-nickel caused by tin reflow treatment andeffective for corrosion resistance, leading to insufficient weldabilityand corrosion resistance. With the increasing amount of tin plated,weldability and corrosion resistance are increased. If the amount of tinplated exceeds 1 g/m², irrespective of further improvements inweldability and corrosion resistance, the cost of manufacture becomestoo high to meet the economic requirement for welded can-forming steel.For this reason, the tin plated on the first layer is limited to abuild-up of 0.1 to 1 g/m², and preferably 0.3 to 0.6 g/m² according tothe present invention.

According to a first embodiment of the present invention, a tin fusionor reflow treatment is carried out at the end of tin plating to form asecond layer of iron-tin-nickel alloy. The tin reflow may be effected byheating to above the melting point of tin, for example, by electricresistance heating, induction heating, external heating and otherconventional techniques. The desired quality is achievable by any ofthese techniques. This second layer is a pinhole-free uniform coatingwhich fully protects the underlying steel and contributes to asubstantial part of improvement in corrosion resistance. Unlike iron-tinalloy layers formed by tin reflow in conventional tinplate manufacture,the second layer of iron-tin-nickel alloy according to the presentinvention is highly resistant to corrosive attack by can contents orfoodstuffs due to the inclusion of nickel. The iron-tin-nickel alloylayer resulting from tin reflow treatment is formed by a tin reflowtreatment in a necessary and sufficient amount (or thickness) as long asthe amount of tin plated previously is within the above-specified rangeof 0.1 to 1 g/m². Within this range of tin plating, alloying a part orall of the tin plating does not affect weldability and corrosionresistance.

On the second layer of iron-tin-nickel alloy thus formed by tin platingfollowed by reflowing, is formed a third layer consisting essentially ofmetallic chromium and hydrated chromium oxide by effecting anelectrolytic chromate treatment. The third layer is the top coat whichis required to maintain firm adhesion of lacquer, but can adverselyaffect weldability if it is too thick.

The chromate treatment may be carried out through cathodic electrolysisin a solution containing one or more of chromic acid, chromates, anddichromates. According to the present invention, the total amount ofmetallic chromium and hydrated chromium oxide is limited to the range of5 to 20 mg/m² calculated as elemental chromium. With total amounts ofless than 5 mg/m², adhesion of lacquer to the chromated layer is poor sothat a lacquer film may be readily separated at the location of defectstherein, to the detriment of the corrosion resistance of the lacquerfilm. If the chromated layer is more than 20 mg/m² (calculated aselemental chromium) the increased electric resistance of the chromatedlayer becomes a bar against welding. For this reason, the thirdchromated layer according to the present invention should have acombined chromium content of 5 to 20 mg/m², and preferably 7 to 15mg/m².

Because of the three layer structure comprising the first layer ofiron-nickel alloy, the second layer of iron-tin-nickel alloy formed bytin plating on the first layer followed by tin reflowing, and the thirdchromate layer on the second layer, the surface-treated steel strips orsheets according to the first embodiment of the present inventionexhibit improved weldability and corrosion resistance after lacqueringand are thus very suitable to form food cans by electric resistancewelding.

The process according to the first embodiment of the present inventionis successful in providing thinly coated tin plates having improvedweldability and corrosion resistance. Through a series of experiments,we have found that among steel strips surface treated by the processaccording to the first embodiment of the present invention, some are notsatisfactory in corrosion resistance when combined with certain cancontents or foodstuffs. Continuing further research, we have found thatcorrosion resistance changes depending on the composition of the thirdlayer, i.e., chromate film and that the tin reflow treatment is notnecessarily needed to form the second layer. The former finding will bedescribed in more detail. Although the combined chromium content in thethird layer consisting essentially of metallic chromium and hydratedchromium oxide is limited in the first embodiment, we have found thatthose treated steel strips showing poor corrosion resistance have achromate film containing a smaller amount of metallic chromium.

The corrosion resistance after lacquering and weldability ofsurface-treated steel strips were examined by controlling the finalelectrolytic chromate treatment to change the respective amounts ofmetallic chromium and hydrated chromium oxide in the chromate film. Theresults are plotted in the graph of FIG. 3, in which X represents theamount of metallic chromium in the third chromate layer and Y representsthe amount of hydrated chromium oxide in the third chromate layercalculated as elemental chromium, both expressed in mg/m². In FIG. 3,the optimum region is a region in which both corrosion resistance afterlacquering and weldability are excellent. When the amount of metallicchromium (X) is more than 2 mg/m², corrosion resistance was excellent.When the amount of metallic chromium (X) is less than 2 mg/m², somesamples showed poor corrosion resistance. It is believed that theadhesion of a chromate film to lacquer increases with the increasingamount of metallic chromium in the chromate film, particularly at aproportion of metallic chromium of more than 2 mg/m² (inclusive). Asseen from FIG. 3, with respect to the total amount of chromium in thechromate film consisting essentially of metallic chromium and hydratedchromium oxide, that is, X+Y, corrosion resistance is poor below 5 mg/m²whereas weldability becomes unsatisfactory above 20 mg/m². The rangedefined by 5≦X+Y≦20 is an optimum range in which both corrosionresistance and weldability are excellent.

According to the second embodiment of the present invention, thefollowing relationships should be met:

2≦X and

5≦X+Y≦20

where X and Y are as defined above.

Next, the tin reflow treatment will be discussed. In the firstembodiment, the formation of the second layer of Fe-Sn-Ni alloy dependson the tin reflow treatment. However, when corrosion resistance afterlacquering is considered, a tin plating is converted into aniron-nickel-tin alloy during baking of the lacquer and this subsequentlyformed alloy layer is equal in effect to the alloy layer previouslyformed by reflowing. Accordingly, the tin reflow treatment is optionalin the step of forming the second layer.

Because of the three layer structure comprising the first layer ofiron-nickel alloy, the second layer of tin or iron-tin-nickel alloy onthe first layer, and the third layer consisting of chromate filmconsisting essentially of controlled amounts of metallic chromium andhydrated chromium oxide on the second layer, the surface-treated steelstrips or sheets according to the second embodiment of the presentinvention consistently exhibit improved weldability and corrosionresistance after lacquering and are thus highly suitable to form foodcans by electric resistance welding.

Examples of the present invention are given by way of illustration andnot by way of limitation.

EXAMPLE I

A conventional steel strip intended for electroplating was cold rolledto a thickness of 0.2 mm and electrolytically cleaned in a usual mannerbefore it was cut into samples designated Nos. 1 to 14. Surface-treatedsteel samples were prepared from these steel samples by the processaccording to the present invention and by similar processes in which atleast one parameter did not fulfill the requirements of the presentinvention. The samples were then tested for weldability and corrosionresistance after lacquering.

A. Formation of the First Layer of Iron-Nickel Alloy

The first layer of iron-nickel alloy was formed on steel samples by oneor a combination of two or more of the following procedures:

(a) nickel plating followed by annealing,

(b) plating of an iron-nickel alloy followed by annealing, and

(c) plating of an iron-nickel alloy.

For instance, a steel strip was cold rolled to a thickness of 0.2 mm andelectrolytically cleaned in a sodium hydroxide solution. The steel stripwas then plated with nickel or an iron-nickel alloy and annealed in anatmosphere of 10% H₂ +90% N₂, that is, the so-called HNX gas atmosphere.The thus annealed strip was further electorolytically cleaned in acaustic soda solution, pickled in a sulfuric acid solution, and thenplated with an iron-nickel alloy. Typical examples of the plating bathsused had the following compositions.

    ______________________________________                                        (a) Nickel plating bath                                                       ______________________________________                                        Nickel sulfate        250    g/1                                              Nickel chloride       45     g/1                                              Boric acid            30     g/1                                              ______________________________________                                    

    ______________________________________                                        (b) Iron-nickel alloy plating bath                                            ______________________________________                                        Iron chloride        20-230  g/1                                              Nickel chloride      30-300  g/1                                              Boric acid           25      g/1                                              ______________________________________                                    

A first layer of iron-nickel alloy was formed on the surface of a steelstrip in this way. For sample Nos. 1 to 7 according to the presentinvention, the first layers formed had a weight ratio of Ni/(Fe+Ni) inthe range between 0.02 and 0.50 and a thickness of 10 to 5,000 Å asshown in Table 1, satisfying the requirements of the invention. Amongsamples for comparison purpose, sample Nos. 9 and 11 had a weight ratioof Ni/(Fe+Ni) of 0.01 and 0.85, respectively, not satisfying therequirement of the invention. Sample No. 10 had a first layer whosethickness is as great as 6,000 Å, exceeding the requirement of theinvention.

It is to be noted that the composition and thickness of the first layerof iron-nickel alloy shown in Table 1 were measured by IMMA.

B. Formation of the Second Layer of Iron-Tin-Nickel Alloy

Tin was deposited on the first layer and a tin fusion or reflowtreatment was carried out to form a second layer of an iron-tin-nickelalloy. A typical example of the tin plating bath used is a halide bathhaving the following composition:

    ______________________________________                                        Tin-plating halide bath                                                       ______________________________________                                        Stannous chloride       60    g/1                                             Acidic sodium fluoride  20    g/1                                             Sodium fluoride         50    g/1                                             Sodium chloride         60    g/1                                             ______________________________________                                    

In this step, sample Nos. 1 to 7 according to the present invention hadtin plated to a coating weight in the range of 0.1 to 1 g/m², i.e.,100-1,000 mg/m², satisfying the requirement of the invention. Amongsamples for comparison purpose, sample No. 13 had tin plated to acoating weight as little as 80 mg/m² and sample No. 14 had tin plated toas much as 2,800 mg/m², not satisfying the requirement of the invention.It is to be noted that sample No. 14 having tin thickly coatedcorresponds to #25 tinplate which is the most thinly coated tinplateamong currently available tinplates.

C. Formation of the Third Layer of Metallic Chromium and HydratedChromium Oxide by Electrolytic Chromate Treatment

The tin-plated steel samples were subjected to cathodic electrolysis ina chromate treating bath which typically had the following composition.

    ______________________________________                                        Chromate treating bath                                                        ______________________________________                                        Chromic anhydride      5     g/1                                              Sodium dichromate      20    g/1                                              Sulfuric acid          0.1   g/1                                              ______________________________________                                    

With respect to the total amount of metallic chromium and hydratedchromium oxide in the third layer formed by this electrolytic chromatetreatment, sample Nos. 1 to 7 according to the invention all satisfiedthe requirement of the invention in the range of 5 to 20 mg/m²calculated as metallic chromium as shown in Table 1. Among samples forcomparison purpose, sample No. 8 had a total chromium amount as small as4 mg/m² and sample No. 12 had a total chromium amount as large as 22mg/m², not satisfying the requirement of the invention.

Test specimens were cut from the thus obtained samples to examine theirproperties.

Weldability and corrosion resistance after lacquer coating wereevaluated as follows.

Weldability

A copper wire having a diameter of about 1.5 mm was used as a weldingelectrode. A specimen was rounded to place one edge on the mating edgeunder pressure. While the copper wire was moved along the overlappingedge, electric resistance welding was conducted at a welding rate of 40m per minute. Optimum ranges for electric current and pressure appliedduring welding were sought within which a weld zone having sufficientstrength could be produced without so-called splashes. The presence ofthese ranges ensures the weldability of specimens.

The strength of a weld zone was determined by the so-called peel test inwhich a V-shaped notch was cut in one end of the rounded specimen acrossthe weld line. The bevelled portion of the overlapping edge was pulledwith a pliers toward the other end. The strength required is such thatthe specimen may not be fractured at the weld in this process.

Corrosion Resistance After Lacquering

A specimen was coated with an epoxy-phenol lacquer to a thickness of 4.5microns (μm) and cuts were formed through the lacquer film to theunderlying steel substrate with a fine cutting knife. The specimen wasdrawn to 5 mm through an Erichsen machine.

The thus treated specimen was evaluated for corrosion resistance byimmersing it for 96 hours in a deaerated solution of 1.5% citric acidand 1.5% salt water in 1:1 admixture. The steel underlying the lacquerfilm was evaluated for corrosion by determining the distance of thelacquer film separated from the cross-cut and the quantity of irondissolved out from the cross-cut.

The results of evaluation on welded specimens and lacquer coatedspecimens originating from sample Nos. 1 and 14 are shown in Table 1.Symbols used to evaluate weldability and corrosion resistance afterlacquer coating in Table 1 have the following meanings.

    ______________________________________                                        Weldability                                                                   Symbol      Optimum welding range                                             ______________________________________                                        O           present                                                           X           absent                                                            ______________________________________                                    

    ______________________________________                                        Corrosion resistance after laquer coating                                                Maximum distance of paint film                                     Symbol     separated from cross-cut                                           ______________________________________                                        O          0                                                                  Δ    0-0.5 mm                                                           X          more than 0.5 mm                                                   ______________________________________                                    

                                      TABLE 1                                     __________________________________________________________________________                                               Weldability                                                                         Corrosion resistance                                       Tin Coat-                                                                           Total Cr in                                                                          (optimum                                                                            after lacquering             Sample                                                                             Iron-nickel alloy layer  ing Weight                                                                          third layer                                                                          welding                                                                             Lacquer                                                                              Fe dissolved          No.  Procedure  Ni/(Fe + Ni)                                                                         Thickness (Å)                                                                    (mg/m.sup.2)                                                                        (mg/m.sup.2)                                                                         range)                                                                              Separation                                                                           out                   __________________________________________________________________________                                                            (mg)                  1    Fe--Ni alloy plating                                                                     0.40    600   200   10     o     o      3                     2    Fe--Ni alloy plating                                                                     0.03    20    500   18     o     o      2                     3    Ni plating/annealing                                                                     0.02-0.3                                                                             1000   150    6     o     o      4                     4    Ni plating/annealing                                                                     0.02-0.2                                                                              700   650   12     o     o      3                     5    Fe--Ni alloy plating/                                                                     0.02-0.25                                                                           4000   350    9     o     o      3                          annealing                                                                6    Fe--Ni alloy plating/                                                                     0.02-0.45                                                                           2500   420   15     o     o      2                          annealing                                                                7    Ni plating/annealing/                                                                    0.02-0.3                                                                              100   550    8     o     o      4                          Fe--Ni alloy plating                                                      8*  Fe--Ni alloy plating                                                                     0.2     800   900    4     o     x      175                    9*  Fe--Ni alloy plating                                                                     0.01    150   250   18     x     Δ                                                                              200                   10*  Fe--Ni alloy plating/                                                                    0.02-0.2                                                                             6000   380   12     o     Δ                                                                              180                        annealing/                                                                    Fe--Ni alloy plating                                                     11*  Fe--Ni alloy plating/                                                                    0.85   1000   540   15     o     x      190                        annealing/                                                                    Fe--Ni alloy plating                                                     12*  Ni plating/annealing                                                                     0.02-0.2                                                                             1000   600   22     x     o      3                     13*  Ni plating/annealing                                                                     0.02-0.3                                                                              300    80   12     o     Δ                                                                              270                   14*  --         --     --     2800   6     o     Δ                                                                              50                    __________________________________________________________________________     *comparative examples                                                    

In Table 1, for comparative sample Nos. 8-14, those numerical values notsatisfying the requirements of the invention are underlined. As seenfrom the results of weldability and corrosion resistance afterlacquering of surface-treated steel samples shown in Table 1, sampleNos. 1 to 7 satisfying all the requirements of the invention exhibitsuperior weldability and corrosion resistance after lacquering ascompared with, for example, sample No. 14 corresponding to #25 tinplatealthough the amount of tin plated is less than one third of that ofsample No. 14. These improvements are based on the surface structure ofmulti-layer construction comprising the first layer of iron-nickelalloy, the second layer of iron-tin-nickel alloy which is uniform andfree of pinholes, and the third layer formed by controlled chromatetreatment. Particularly, the quantity of iron dissolved out from across-cut is in the range between 2 and 4 mg which is in contrast tocomparative samples, proving the improved adhesion of a lacquer film tosurface-treated steel and the improved corrosion resistance to which thesecond layer particularly contributes. Conversely, comparative sampleswhich do not satisfy at least one of the requirements of the inventionare inferior to the samples of the invention in weldability and/orcorrosion resistance, and particularly, are prone to considerablecorrosion at cross-cuts.

As seen from the above example, since the surface-treated steel strip orsheet suitable for electric resistance welding according to the firstembodiment of the present invention is prepared by forming a first layerof iron-nickel alloy on a steel strip, depositing tin on the firstlayer, causing the tin to reflow to form a second layer ofiron-tin-nickel alloy, and effecting an electrolytic chromate treatmentto form a third chromated layer on the second layer, thereby forming asurface structure of multi-layer construction while specificallycontrolling the composition and thickness of the first layer and thebuild-ups of the second and third layers, weldability and corrosionresistance after lacquering are significantly improved as well as theadhesion of a lacquer film to the steel. Thus, the surface-treated steelstrip or sheet according to the present invention satisfies all theabove-mentioned requirements for steel material from which welded foodcans are formed.

EXAMPLE II

A conventional steel strip intended for electroplating was cold rolledto a thickness of 0.2 mm and electrolytically cleaned in a usual mannerbefore it was cut into samples designated Nos. 21 to 34. Surface-treatedsteel samples were prepared from these steel samples by the processaccording to the present invention (Nos. 21-25) and by similar processesin which at least one parameter did not fulfill the requirements of thepresent invention (Nos. 26-34). The samples were then tested forweldability and corrosion resistance after lacquering.

A. Formation of the First Layer of Iron-Nickel Alloy

The first layer of iron-nickel alloy was formed on steel samples by oneor a combination of two or more of the following procedures:

(a) nickel plating followed by annealing,

(b) plating of an iron-nickel alloy followed by annealing, and

(c) plating of an iron-nickel alloy.

For instance, a steel strip was cold rolled to a thickness of 0.2 mm andelectrolytically cleaned in a sodium hydroxide solution. The steel stripwas then plated with nickel or an iron-nickel alloy and annealed in anatmosphere of 10% H₂ +90% N₂, that is, the so-called HNX gas atmosphere.The thus annealed strip was further electrolytically cleaned in a sodiumhydroxide solution, pickled in a sulfuric acid solution, and then platedwith an iron-nickel alloy.

Typical examples of the plating baths used had the followingcompositions.

    ______________________________________                                        (a) Nickel plating bath                                                       ______________________________________                                        Nickel sulfate        250    g/1                                              Nickel chloride       45     g/1                                              Boric acid            30     g/1                                              ______________________________________                                    

    ______________________________________                                        (b) Iron-nickel alloy plating bath                                            ______________________________________                                        Iron chloride        20-230  g/1                                              Nickel chloride      30-300  g/1                                              Boric acid           25      g/1                                              ______________________________________                                    

A first layer of iron-nickel alloy was formed on the surface of a steelstrip in this way. For sample Nos. 21 to 25 according to the presentinvention, the first layers formed had a weight ratio of Ni/(Fe+Ni) inthe range between 0.02 and 0.50 and a thickness of 10 to 5,000 Å asshown in Table 1, satisfying the requirements of the invention. Amongsamples for comparison purposes, sample Nos. 30, 31, and 32 had a weightratio of Ni/(Fe+Ni) of 0.01, 0, and 0.85, respectively, not satisfyingthe requirement of the invention. Sample No. 33 had a first layer whosethickness is as great as 6,000 Å, exceeding the requirement of theinvention.

It is to be noted that the composition and thickness of the first layerof iron-nickel alloy shown in Table 2 were measured by IMMA.

B. Formation of the Second Layer

Tin is deposited on the first layer to form a second layer of tinthereon. Optionally, tin plating is followed by a tin fusion or reflowtreatment to form a second layer of iron-tin-nickel alloy on the firstlayer. The tin reflowing is not necessarily needed because conversioninto such an alloy layer will take place during subsequent baking oflacquer. The corrosion resistance after lacquering is the corrosionresistance of steel at the end of lacquering, that is, at the end oflacquer baking. For this reason, the previous tin reflowing is notnecessarily needed. A significant improvement in corrosion resistance isachievable when the second layer is formed simply by tin plating withoutreflowing and a corresponding alloy layer is subsequently formed duringbaking of lacquer. A typical example of the tin plating bath used is ahalide bath having the following composition:

    ______________________________________                                        Tin-plating halide bath                                                       ______________________________________                                        Stannous chloride       60    g/1                                             Acidic sodium fluoride  20    g/1                                             Sodium fluoride         50    g/1                                             Sodium chloride         60    g/1                                             ______________________________________                                    

In this step, sample Nos. 21 to 25 according to the present inventionhad tin plated to a coating weight in the range of 0.1 to 1 g/m², i.e.,100-1,000 mg/m², satisfying the requirement of the invention. Amongsamples for comparison purposes, sample No. 34 had tin plated to acoating weight as little as 50 mg/m² and sample No. 31 had tin plated toas much as 2,800 mg/m², not satisfying the requirement of the invention.It is to be noted that sample No. 31 having tin thickly coatedcorresponds to #25 tinplate which is the most thinly coated tinplateamong currently available tinplates.

C. Formation of the Third Layer of Metallic Chromium and HydratedChromium Oxide by Electrolytic Chromate Treatment

The tin-plated steel samples were subjected to cathodic electrolysis inchromate treating baths which typically had the following compositions.

    ______________________________________                                               Bath I                                                                        CrO.sub.3      20    g/1                                                      Na.sub.2 Cr.sub.2 O.sub.7                                                                    5     g/1                                                      H.sub.2 SO.sub.4                                                                             0.2   g/1                                                      Bath II                                                                       CrO.sub.3      15    g/1                                                      NaF            1.5   g/1                                                      Bath III                                                                      CrO.sub.3      60    g/1                                                      Bath IV                                                                       Na.sub.2 Cr.sub.2 O.sub.7                                                                    55    g/1                                                      Cr.sub.2 O.sub.3                                                                             5     g/1                                               ______________________________________                                    

With respect to the total amount (X+Y) of metallic chromium and hydratedchromium oxide in the third layer formed by this electrolyric chromatetreatment, sample Nos. 21 to 25 according to the invention all satisfiedthe requirement of the invention that the amount of metallic chromium Xbe equal to or more than 2 mg/m² and the total chromium amount (X+Y) isin the range between 5 and 20 mg/m² (inclusive) as shown in Table 2.Among samples for comparison purposes, sample Nos. 22, 27, and 31 had ametallic chromium amount as small as 0, 1, and 0 mg/m² and sample Nos.28 and 29 had a total chromium amount as large as 25 and 30 mg/m², notsatisfying the requirement of the invention.

Test specimens were cut from the thus obtained samples to examine theirproperties.

Weldability and corrosion resistance after lacquering were evaluated asfollows.

Weldability

A copper wire having a diameter of about 1.5 mm was used as a weldingelectrode. A specimen was rounded to place one edge on the mating edgeunder pressure. While the copper wire was moved along the overlappingedge, electric resistance welding was conducted at a welding rate of 40m per minute. Optimum ranges for electric current and pressure appliedduring welding were sought within which a weld zone having sufficientstrength could be produced without so-called splashes. The presence ofthese ranges ensures the weldability of specimens.

The strength of a weld zone was determined by the so-called peel test inwhich a V-shaped notch was cut in one end of the rounded specimen acrossthe weld line. The bevelled portion of the overlapping edge was pulledwith a pliers toward the other end. The strength required is such thatthe specimen may not be fractured at the weld in this process.

Corrosion Resistance After Lacquer Coating

A specimen was coated on one surface with an epoxy-phenol lacquer to acoating weight of 50 mg/dm² and sealed at the edges and oppositesurface.

The thus treated specimen was evaluated for corrosion resistance byimmersing it totally in a test solution and then keeping it halfimmersed at 55° C. for 18 days. After 18-day immersion, the specimen wasremoved from the solution and the upper half of the specimen which hadbeen above the solution level was observed for corrosion under thelacquer film.

The test solutions used were commercially available grapefruit juice,tomato juice, and milk.

The results of evaluation on welded specimens and lacquered specimensoriginating from samples Nos. 21 to 34 are shown in Table 2. Symbolsused to evaluate weldability and corrosion resistance after lacqueringin Table 2 have the following meanings.

    ______________________________________                                        Weldability                                                                   Symbol      Optimum welding range                                             ______________________________________                                        O           present                                                           X           absent                                                            ______________________________________                                    

    ______________________________________                                        Corrosion resistance after lacquering                                                   Blister of lacquer film on upper                                    Symbol    half of specimen above solution                                     ______________________________________                                        O         no blister                                                          Δ   some blister                                                        X         much blister                                                        ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________                                                 Properties                                                                        Corrosion resistance         Iron Nickel alloy layer    Tin Coat-                                                                           Tin                                                                              Chromate     after lacquering in          Sample                Thick-                                                                             ing Weight                                                                          re-                                                                              treatment                                                                              Weld-                                                                             Grapefruit                                                                          Tomato                 No. Procedure  Ni/(Fe + Ni)                                                                         ness (Å)                                                                       (mg/m.sup.2)                                                                        flow                                                                             Bath                                                                             X  Y  ability                                                                           Juice Juice                                                                              Milk              __________________________________________________________________________    21  Ni plating/annealing                                                                     0.02-0.2                                                                             800  600   R  I  2   8 o   o     o    o                 22  Ni plating/annealing                                                                     0.02-0.2                                                                             800  620   R  II 5  10 o   o     o    o                 23  Fe--Ni alloy plating                                                                     0.03    40  350   N  I  8  16 o   o     o    o                 24  Fe--Ni alloy plating/                                                                    0.02-0.3                                                                             3000 500   R  II 6  14 o   o     o    o                     annealing                                                                 25  Ni plating/annealing/                                                                    0.02-0.4                                                                              50  600   N  II 3  10 o   o     o    o                     Fe--Ni alloy plating                                                      26* Ni plating/annealing                                                                     0.02- 0.2                                                                            800  740   R  III                                                                              0  10 o   o     Δ                                                                            x                 27* Ni plating/annealing                                                                     0.02-0.2                                                                             800  650   R  I  1  12 o   o     o    Δ           28* Ni plating/annealing                                                                     0.02-0.2                                                                             800  700   N  II 10 25 x   o     o    o                 29* Ni plating/annealing                                                                     0.02-0.2                                                                             800  800   R  III                                                                              0  30 x   o     x    x                 30* Fe--Ni alloy plating                                                                     0.01   150  500   R  II 6  12 o   o     Δ                                                                            Δ           31* --         --     --   2800  R  IV 0   6 o   o     Δ                                                                            Δ           32* Fe--Ni alloy plating/                                                                    0.85   1000 500   R  II 5  11 x   o     o    Δ               annealing/                                                                    Fe--Ni alloy plating                                                      33* Fe--Ni alloy plating/                                                                    0.02-0.2                                                                             6000 400   R  I  4  13 o   o     o    Δ               annealing/                                                                    Fe--Ni alloy plating                                                      34* Fe--Ni alloy plating                                                                     0.03    40   50   R  I  5  14 x   Δ                                                                             x    x                 __________________________________________________________________________     *comparative examples                                                         R = reflow                                                                    N = no reflow                                                            

In Table 2, for comparative samples Nos. 26-34, those numerical valuesnot satisfying the requirements of the invention are underlined. As seenfrom the results of weldability and corrosion resistance afterlacquering of surface-treated steel samples shown in Table 1, sampleNos. 21 to 25 satisfying all the requirements of the invention exhibitsuperior weldability and corrosion resistance after lacquer coating ascompared with, for example, sample No. 31 corresponding to #25 tinplatealthough the amount of tin plated is less than one third of that ofsample No. 31. These improvements are based on the surface structure ofmulti-layer construction comprising the first layer of iron-nickelalloy, the second layer of iron-tin-nickel alloy which is uniform andfree of a pinhole, and the third layer formed by controlled chromatetreatment. Conversely, comparative samples which do not satisfy at leastone of the requirements of the invention are inferior to the samples ofthe invention in weldability and/or corrosion resistance.

As seen from the above example, since the surface-treated steel strip orsheet suitable for electric resistance welding according to the presentinvention is prepared by forming a first layer of iron-nickel alloy on asteel strip, depositing tin on the first layer to form a second layer oftin, optionally causing the tin to reflow to form a (converted) secondlayer of iron-tin-nickel alloy, and effecting an electrolytic chromatetreatment to form a third chromated layer on the second layer, therebyforming a surface structure of multi-layer construction whilespecifically controlling the composition and thickness of the firstlayer and the build-ups of the second and third layers, weldability andcorrosion resistance after lacquering are significantly improved as wellas the adhesion of a lacquer film to the steel. Thus, thesurface-treated steel strip or sheet according to the present inventionsatisfies all the above-mentioned requirements for steel material fromwhich welded food cans are formed.

What is claimed is:
 1. Process for preparing a surface-treated steelstrip adapted for electric resistance welding, comprising the stepsofforming a first layer of iron-nickel alloy on a steel strip, saidfirst layer having a weight ratio of Ni/(Fe+Ni) in the range between0.02 and 0.50 and a thickness of 10 to 5,000 angstroms, forming a secondlayer of tin or iron-tin-nickel alloy on said first layer by tin platingto a coating weight of 0.1 to 1 g/m² of tin and forming a third layer onsaid second layer by effecting an electrolytic chromate treatment, saidthird layer consisting essentially of metallic chromium and hydratedchromium oxide in a total amount of 5 to 20 mg/m² calculated aselemental chromium.
 2. The process according to claim 1 wherein saidfirst layer of iron-nickel alloy is formed by (1) plating the steelstrip with nickel followed by annealing, (2) plating the steel stripwith an iron-nickel alloy followed by annealing, (3) plating the steelstrip with an iron-nickel alloy, or (4) a combination of the foregoings.3. The process according to claim 1 wherein said first layer ofiron-nickel alloy has a weight ratio of Ni/(Fe+Ni) in the range between0.05 and 0.20.
 4. The process according to claim 1 or wherein said firstlayer of iron-nickel alloy has a thickness of 100 to 1,500 angstroms. 5.The process according to claim 1 wherein tin plating is effected in abath selected from halide baths, ferrostan baths, and alkaline bathscontaining an effective concentration of tin.
 6. The process accordingto claim 1 wherein tin plating is effected to a coating weight of 0.3 to0.6 g/m² of tin.
 7. The process according to claim 1 wherein tinreflowing is effected by heating.
 8. The process according to claim 1wherein the electrolytic chromate treatment is cathodic electrolysis ina bath containing at least one selected from chromic acid, chromates,and dichromates.
 9. The process according to claim 1 wherein said thirdlayer consists essentially of metallic chromium and hydrated chromiumoxide in a total amount of 7 to 15 mg/m² calculated as elementalchromium.
 10. The process according to claim 1 wherein the electrolyticchromate treatment is controlled such that at least 2 mg/m² of metallicchromium is present in the third layer.
 11. The process according toclaim 1, and causing the tin to reflow after formation of said secondlayer.
 12. Process for preparing a surface-treated steel strip adaptedfor electric resistance welding, comprising the steps offorming a firstlayer of iron-nickel alloy on a steel strip, said first layer having aweight ratio of Ni/(Fe+Ni) in the range between 0.02 and 0.50 and athickness of 10 to 5,000 angstroms, forming a second layer of tin oriron-tin-nickel alloy on said first layer by tin plating to a coatingweight of 0.1 to 1 g/m² of tin, and forming a third layer on said secondlayer by effecting an electrolytic chromate treatment, said third layerconsisting essentially of metallic chromium and hydrated chromium oxide,wherein the following relationships:2≦X and 5≦X+Y≦20 are met providedthat X represents the amount of metallic chromium in the third layer andY represents the amount of hydrated chromium oxide in the third layercalculated as elemental chromium, both expressed in mg/m².
 13. Theprocess according to claim 12 wherein said first layer of iron-nickelalloy is formed by (1) plating the steel strip with nickel followed byannealing, (2) plating the steel strip with an iron-nickel alloyfollowed by annealing, (3) plating the steel strip with an iron-nickelalloy, or (4) a combination of the foregoings.
 14. The process accordingto claim 12 wherein said first layer of iron-nickel alloy has a weightratio of Ni/(Fe+Ni) in the range between 0.05 and 0.20.
 15. The processaccording to claim 12 wherein said first layer of iron-nickel alloy hasa thickness of 100 to 1,500 angstroms.
 16. The process according toclaim 12 wherein tin plating is effected in a bath selected from halidebaths, ferrostan baths, and alkaline baths containing an effectiveconcentration of tin.
 17. The process according to claim 12 wherein tinplating is effected to a coating weight of 0.3 to 0.6 g/m² of tin. 18.The process according to claim 12 wherein tin reflowing is effected byheating.
 19. The process according to claim 12 wherein the electrolyticchromate treatment is cathodic electrolysis in a bath containing atleast one selected from chromic acid, chromates, and dichromates. 20.The process according to claim 12 wherein said third layer consistsessentially of metallic chromium and hydrated chromium oxide in a totalamount of 7 to 15 mg/m² calculated as elemental chromium.
 21. Theprocess according to claim 12, and causing the tin to reflow afterforming said second layer.